In recent years, as an accumulator device applicable to usage that requires high energy density and high output characteristics, attention has been focused on a lithium ion capacitor.
Such a lithium ion capacitor is often used in an environment where a temperature change is considerable. Thus, a reduction in initial capacity and energy density of the lithium ion capacitor under a low-temperature environment has been a major problem.
Specifically, in a low-temperature region of around −30° C., viscosity of an electrolyte solution increases and thus mobility of electrolyte ions decreases in pores of activated carbon constituting a positive electrode active material. This causes such problems that charge and discharge capacity significantly decreases and internal resistance increases. Thus, in order to increase the mobility of electrolyte ions at low temperatures, activated carbon having a large pore volume (cc/g) has been used as a positive electrode active material. However, although the internal resistance is decreased at low temperatures by using the activated carbon having a large pore volume (cc/g), the use of such an activated carbon alone decreases density of the positive electrode, thus causing a problem that capacity per volume (F/cc) becomes low.
On the other hand, in order to achieve further improvement in performance of an electric double layer capacitor under a low-temperature environment, there is proposed a technique for improving charge and discharge characteristics and internal resistance characteristics by using activated carbon having a pore diameter determined by a BJH method within a range of 1.0 to 1.5 nm and a peak value of pore volume within a range of 0.020 to 0.035 cm3/g (see, for example, Patent Literature 1).
However, the activated carbon descried in Patent Literature 1 has a BET specific surface area as small as 1500 to 2200 m2/g. Thus, using such activated carbon as a positive electrode active material in the lithium ion capacitor causes a problem that the charge and discharge capacity decreases even at normal temperature.
Further, as a positive electrode active material for lithium ion capacitor, there is proposed activated carbon having the BET specific surface area of 1500 to 3000 m2/g, a ratio (A) of a volume of pores having a pore diameter of not lower than 0.6 nm and lower than 1 nm to that having a pore diameter of not lower than 0.6 nm and not more than 200 nm within a range of 0.48≤A≤0.70, a ratio (B) of a volume of pores having a pore diameter of not lower than 1 nm and not more than 6 nm to that having a pore diameter of not lower than 0.6 nm and not more than 200 nm within a range of 0.20≤B≤0.52, and a total pore volume within a range of 1.21 cc (mL)/g to 1.62 cc/g (See Patent Literature 2).
However, the positive electrode active material described in the prior art document 2, which has the BET specific surface area of 1500 to 3000 m2/g and the ratio (A) of the volume of pores having a pore diameter of not lower than 0.6 nm and lower than 1 nm within the range of 0.48≤A≤0.70, has an insufficient pore volume in a range of pore diameter necessary for increasing the charge and discharge capacity at around −30° C. This causes a problem that a lowering rate of the charge and discharge capacity at −30° C. with respect to that at normal temperature increases.
Further, in either of the above prior art documents, there is no description relating to pore characteristics of an electrode incorporated into the lithium ion capacitor. Thus it has been unclear as to what kinds of pore characteristics are preferable in the electrode that is constituted by the positive electrode active material as well as other auxiliary materials such as a conductive aid and a binder.